In the food processing industry, spiral conveyors having a generally vertical axis, are often employed in view of the economies in respect of floor space. These spiral conveyors are of particular advantage where the product reguires lengthy exposure to a controlled environment, such as drying, heating, cooking, cooling and freezing. Accordingly the spiral conveyors are generally housed within an insulated enclosure so that the environment may be controlled. Within this controlled environment, often air is treated and then passed across the product in order to subject the product to the desired conditions. However it is a disadvantage of these known apparatus, that the air stream delivered to the conveyor, is not uniform in flow rate along the height of the conveyor nor angularly about the conveyor. Known apparatus have attempted to overcome this problem by internal or external baffles, or tuning vanes. However these known apparatus have not been totally successful and are generally expensive due to the requirement for baffles and vanes.
The above discussed spiral conveyors are disclosed in U.S. Pat. No. 4271683, U.S. Pat. No. 4363263, U.S. Pat. No. 4480535 (AU 15751/83), AU 21748/83 and AU 11905/83.
Of this prior art, the most relevant is U.S. Pat. No. 4480535, which relates to an "Apparatus for Air Treatment of Products". This document discloses an apparatus to freeze foodstuffs, which apparatus has as its inventive feature a perforated cylinder which continuously varies the air supply over the length of the spiral conveyor. No consideration is given to causing the air supply to rotate about the axis of the conveyor.